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2004-01-30
I haven't been able to write much lately, but here's a bit of thought for you.
In the Boston Review, Gary Marcus, a professor of psychology at NYU, has an article about new views of the genetic encoding of the brain. The article is aimed at a lay audience, so if you have any background in the topic at all, you'll have to slog through quite a bit of an introduction to basic genetics and how genes work, how they're turned on and off, etc.
So after including all that, Marcus says that one of the reasons people in the past have leaned so heavily on so-called "nurture" theories of brain development is that the number of genes encoded in the genome is really astoundingly small. 30,000 is the number he gives. (Has it been pinned down recently? I haven't been keeping up.)
But, he says, genes are like computer code--they can be turned on and off in lots of combinations. Also, he says, if the complexity of the brain can't be explained by genetics, then surely the complexity of other parts of the body can't either.
(Is this really an argument people encounter? It seems kind of straw-mannish to me. I certainly never encountered any resistance to the idea that the brain's structure--its gross structure, anyway--arose any differently than other organs. And certainly the small number of genes is puzzling, but I never thought of it as having any deeper philosophical ramifications or implications about the so-called "nature/nurture" debate. Anyway.)
Marcus' solution to this alleged conundrum (and I hope his whole book isn't based on this idea) is that the the information in the genome is just highly compressed according to a certain algorithm. Like GIFs or JPGs or MP3s, a huge amount of data is pressed into a very small space by means of various shortcuts and encodings.
Laurie Anderson has a song on her album Home of the Brave whose lyrics, in their entirety, are as follows:
What Fassbinder film is it?
The one-armed Man walks into a flower shop and says:
"What flower expresses,
'Days go by,
And they just keep going by endlessly,
Pulling you Into the future.
Days go by
Endlessly,
Endlessly pulling you
Into the future'?"
And the florist says:
"White Lily."
Now there is what you would call a compression scheme to end all compression schemes. Or perhaps the mother of all compression schemes. A complex and emotionally charged sentiment comes to be represented by a single flower!
I don't mean to be flip. Anderson is talking about something more profound than a simple mapping of emotions to flowers, which is all an encoding or compression algorithm is. But it does make a very basic point about compression algorithms that Marcus, as far as I can tell, completely misses: an arbitrarily large amount of data can be compressed losslessly into an arbitrarily small amount of space by an arbitrarily complicated compression algorithm. I can compress the entire contents of the Encyclopedia Britannica to the single character '&' by means of the mapping "the character '&' represents [the contents of the Encyclopedia Britannica]."
"But that's a trivial mapping!" you might say. Well, sure, but the point I'm trying to make is that a compression algorithm doesn't create data where no data previously existed. It merely offloads the expense of writing out all the data explicitly to some other format or formula. Computer scientists used to use compression algorithms not because it allowed them to arbitrarily shrink data, but because saving data on hard disk or sending it through a network was extremely expensive relative to computing time. So they made a trade-off; they spent some computer cycles to decode encoded data and some hard disk to store their algorithm, and in return they didn't use up quite so much hard disk or network bandwith for their data files. If they only had one small data file, it wouldn't be worth it, because the compressed data file plus the decoding program would take up more space than the decompressed file, and you have to waste processor time on top of that.
So characterizing the genome as a compressed data file isn't the whole story. Obviously, if you took a strand of DNA and laid it on a slab, nothing would happen, just like a GIF file is useless without a GIF viewer. And if you use the wrong decoder, all you get is gibberish. In order to characterize the information encoded in the genome, you have to characterize the decoder as well.
Well, I'll cut to the chase. What is the decoder in this case? The environment in which the organism develops. Marcus wants to say that the decoder is "the cell," by which he means...a fertilized egg? Different cells interpret the genome in different ways, and a fertilized egg is the only cell I can think of that generates an organism when you stick a genome in it. (This is how cloning works.) But how a cell--even a fertilized egg--interprets a genome is not fully specified until you specify the environment in which the cell develops. Add some toxins or something--change the decoder--and all you get is gibberish.
And you know, I don't think the genome is necessarily fabulously efficient in its compression. It could just be that when the place you're offloading the data to is the universe at large, there's an awful lot of space to cheaply store that data. (Well, really, what constitutes "efficient," anyway? It depends on the relative costs of data storage.) I'd have to think about it more, but it seems to me that the place where the complexity is really stored is the folding of proteins, which is governed quite a bit by the laws of physics and not so much by mechanical operations within the cell or anything like that. If nothing else, that step kind of resembles a hash function: change a single amino acid, and your protein might fold in a wildly different--and unpredictable--way. Okay, I'm just speculating at this point, but it seems like a possibility. I'll think about it more.
But anyway, you can see that Marcus's characterization of the genome as compressed data says basically absolutely nothing, and illuminates the so-called "nature/nurture debate" not a glimmer, except perhaps that the compression metaphor makes it easier to explain why the so-called "nature/nuture debate" is completely senseless and incoherent. (Maybe that's all he's trying to do. Certainly much of the general public seems quite credulous of the "blueprint" model and the "nature/nurture debate." I've even heard trained geneticists say "This gene is the gene for X.")
Marcus seems to get this on some level, but wants, perhaps because he is a geneticist, to keep prioritizing nature over nurture. Nature via nurture? No, it's nuture via nature! Well, I guess it's the same old foreground/background question. Whatever floats your inquiry.
But I have to say, if "the extent to which (and ways in which) genes make it possible for experience to rewire the brain" is a question that has never been seriously asked before, maybe I should quit my job and become a geneticist or an evolutionary psychologist.Obviously what makes humans unique is not that they have a supercomputer's worth of processing power atop their spinal column, but that they are so damn adaptable. The interesting question about humans' evolutionary history has always seemed to me not to be how various "hard-coded" traits were selected for, but how adaptability itself was selected for. Thus many "evolutionary psychology" results seems to me like missing the forest for the trees.
Oh, by the way, the Fassbinder film is Berlin Alexanderplatz.
--Melissa O, at 02:58
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